Field of the Invention:
The invention relates to a method for synchronizing the reproduction of a digital signal sent by a transmitter to a plurality of mobile audio terminals, wherein the mobile audio terminals each comprise a signal receiver, a First-In-First-Out memory connected to the signal receiver and an audio reproduction unit connected to the First-In-First-Out memory.
Until now, digital wireless connections have been used for hearing aids primarily to allow adjustment of the operating parameters of the hearing aid, for instance the volume, from a hand-held device. Recently, digital wireless transmission protocols have also been used to stream audio data to the hearing aid from a source designed for this purpose, for instance from a television set or a telephone. In this case, the source has a wired signal connection to a transmitter, which emits a digital radio signal that can be received by a hearing aid equipped with a suitable signal receiver.
To give a particular surround sound effect, it is advantageous here to use a separate hearing aid for each ear and to encode the signal sent by the transmitter in a suitable form for stereo reception. In this case, the hearing aids have two operating modes: in standard mode, a microphone or microphone system of each hearing aid picks up acoustic signals from the surroundings, which are amplified in the hearing aid and which a loudspeaker reproduces to the user's ear; in streaming mode, a digital radio signal containing encoded audio data is received, and the loudspeaker reproduces the audio data. A hand-held device for controlling the volume or for switching between the operating modes etc. can optionally also be provided here.
Reproduction of the transmitted audio data must proceed preferably synchronously for the left and right ears. If the reproduction is not entirely synchronous, i.e. if there is a slight time offset between the signals for the left ear and the right ear, this is perceived by the user as a shift in the direction from which the sound is coming. A sound signal that is perceived as centered when reproduction is synchronous, for the user of the hearing-aid system has its origin to the right or left of center when synchronization is incorrect. The described spatial asymmetry in perception can arise even for a time offset of the order of about ten audio samples.
Such a time offset does not normally occur in conventional audio reproduction units, because either, for instance as with active stereo headphones, a common receiver is used for receiving, and common time synchronization is used for distribution to the separate stereo reproduction units, or the separate stereo reproduction units are in radio communication with one another for synchronizing their respective time scales, as is the case in home cinema systems for example. In a binaural hearing-aid apparatus, however, there is normally no wired connection between the two hearing aids, and also, for reasons of battery capacity, it is usually undesirable to have wireless synchronization of the time scales of said hearing aids. The timing of the reproduction of received digital audio data is thus performed solely on the basis of the internal clock in each hearing aid.
In a binaural hearing-aid system, however, the two internal clocks of the respective hearing aids can exhibit very slight differences. For instance if quartz crystals are used for timing, such differences can arise from component variation or very slight differences in dimensions resulting from manufacturing tolerances. In this case, the differences in timing are typically in the range of 0.001% to 0.01%. Since the audio data is normally not transmitted in real time but is combined by the transmitter into data packets at significantly higher data rates than the reproduction data rate, the data received in each separate hearing aid is buffered in a memory, and therefore the reproduction synchronization depends solely on the respective internal clocks of the hearing aids. For a case in which the timing differs by 0.001% and the sample rate of the audio signal is 16 kHz, then after a reproduction of one minute, an offset of 9.6 samples can accumulate, which corresponds to a time offset of 0.6 ms between the two ears, and which a hearer can already perceive.